This invention relates to fluid handling equipment, and more particularly to arrangements for sealing closures for pressure vessels, valves and pumps.
In fluid handling equipment such as valves and pumps, it is common for a stem or shaft to penetrate the fluid housing in order to accomplish the movement of internal closures or pieces to control the pressure or flow, for example. To accommodate the movement of the stem, clearance must be provided between the stem and the aperture through which it penetrates the housing. To avoid leakage of pressurized fluid from within the housing, it is well known to provide a resilient physical barrier, or packing, between the stem and the portion of the housing or closure member that defines the aperture for the stem.
In a valve of conventional design, the packing is located in a stuffing box surrounding the stem or shaft, and a collar or external gland is mounted in the valve bonnet for compressing the packing in the stuffing box. The collar is typically torqued to apply a high axial force at the upper end of the packing, in order to produce sufficient radial force at the lower end of the packing so that the radial pressure of the packing against the stem is greater than the system pressure within the housing.
The conventional technique for assuring adequate safety margin against fluid leakage through the packing, has been to provide an elongated stuffing box containing additional layers of packing rings or the like. The extra layers of packing require an increase in the axial pressure applied by the collar to achieve the required radial pressure at the lower end of the packing, against the stem. This technique is only marginally effective, in that the higher the stack of packing material along the shaft, the greater the accumulation of friction between the packing and the stem. The fraction of axial pressure applied at the upper end of the stuffing box, that ultimately is manifested as radial pressure against the stem at the lower end of the packing, is greatly diminished. Moreover, the very high axial pressures required on the packing in order to achieve the desired radial sealing pressure at the lower end of the packing, produce radial pressures at the upper end of the packing that can lead to overstressing of the packing rings, high stem loads, and even stem wear and binding. This leads in many cases to rapid packing wear and inconsistent gland-load maintenance Furthermore, the size and strength of the closure components and tightening devices, as well as the motors and actuators for the stem or shaft, must be designed with a very high over capacity relative to the system pressure.